Abstract

This conference contribution will present a novel technology for renewable ash-free fuel production from low-value ash-containing feedstocks such as sludge digestate, wood bark or rice husks that cannot be utilized directly as a heating fuel, because of the high (low melting point) ash content. Biomass Ash Removal (BiAR) process introduces the low-temperature solvolysis in cheap solvents to transform the organic matter into a liquid state, allowing its separation from the solid ash by a simple filtration or sedimentation. After the BiAR process, all tested feedstocks reduced their ash content below threshold admitted for heating biomass set by UNI EN ISO 17225:2014 standard. As anaerobic digestion is capable to recover about 50% of the carbon in form of methane (according with COD balance), BiAR could be used to ‘retrofit’ those installation to recovery the remaining 50% organic carbon leading to a potential 100% carbon, thus energy, recovery. A potential complete compensation of costs associated to dehydration and disposal of digestate could be achieved. The PCT application has been recently filed for the BiAR process (PCT/IT2016/000140).

INTRODUCTION

Circular economy concept means economy generating zero waste, where everything is being reused for some purpose. The ‘Circular Economy Package’ is a set of rules put in place by European Commission in order to stimulate Europe's transition towards a circular economy which will boost global competitiveness, foster sustainable economic growth and generate new jobs. However, despite the efforts, so far many residual streams are still landfilled or disposed by incineration. Waste water sludge is not an exception. Even if anaerobic biodigestion has become widely employed to recover some of the organic carbon in form of energy, the residual digestate still need to be disposed or sent to composting in the best case scenario.

The BiAR process (Biomass Ash Removal) does not compete with other energy recovery systems as it focuses on ash rich feedstocks which cannot be used for energy purposes, or it targets those feedstocks, like sludge or digestate, which have already undergone an energy recovery process. To this extent BiAR can be considered a ‘plug in’ process to retrofit existing wastewater facilities in order to generate additional revenues or reduce costs associated with waste stream disposal.

ENERGY RECOVERY OF BIOMASS

Ash-rich biomass pose severe challenges for any energy recovery because of the low melting point of inorganics. Melting ashes can severely interfere with furnace or boilers causing clogging, fouling and eventually stopping the operations or reducing their performance. Because of that those feedstocks are often set aside or used only in special designed equipment.

Biomass is a very heterogeneous feedstock with different properties among types. In order to have a reference benchmark we decided to look for the standards employed in biomass classification for energy purposes.

Three international standards in the ISO 17225 series entered into force in 2014 which regulate the general (ISO 17225-1) and specific requirements relating to wood pellets (ISO 17225-2) and wood briquettes (ISO 17225-3) (Eubionet3). The specific requirements regulate, among others, the maximum content of ash. The ISO standards also distinguish between three quality categories of pellets for commercial (industrial) and residential applications. In Europe, the Standard EN 14961 establishes ash content limits for wood pellets used for heating purposes. As consequence of the new standards, the project European Pellet Quality Certification created and implemented the ambitious and uniform certification system for pellets in Europe, called ‘ENplus’, which is used both the heat and power markets. The ENplus certification scheme defines 3 pellet quality classes: ENplus A1, ENplus A2, and Enplus B. Ash limits are indicated in Table 1.

Table 1

EN plus – threshold values of ash content and GHV

  EN plus A1 EN plus A2 EN plus B 
Ash (%w) <0.7 <1.2 <2.0 
GHV (Mj/Kg) ≥16.5 ≥16.5 ≥16.5 
  EN plus A1 EN plus A2 EN plus B 
Ash (%w) <0.7 <1.2 <2.0 
GHV (Mj/Kg) ≥16.5 ≥16.5 ≥16.5 

Some domestic pellets stoves, for example, admits only A1 grade pellets containing less than 0.7% wt ash, and even industrial grade wood pellet B class require maximum ash content <2% wt. Obviously feedstocks like bark which contains up to 10% ash or rice husk whose content is almost 15% are often set aside for this purpose.

For this reason there is huge availability of low-value ash-containing feedstocks; sludge digestate whose ash content can reach 35% is also a possible abundant feedstock, with the benefit of being relatively easy to collect as it is concentrated in wastewater treatment plant and not widespread.

BiAR PROCESS

BiAR use solvolysis as an organic-inorganic matter separation process followed by separation of ash from liquefied biomass and then solvent recovery (Figure 1).

Figure 1

Scheme of the BiAR process.

Figure 1

Scheme of the BiAR process.

Liquefaction of biomass is performed by solvolysis route at moderate temperatures (approx. 200°C) in presence of methanol or methanol-glycerol mixture and an acidic homogeneous catalyst in a stirred autoclave. Reaction takes places at elevated autogenic pressure of alcohols at elevated temperature. After reaction time of approx. 30 minutes the vessel is cooled down and the product mixture is filtered to separate ash-rich solid residues from the depolymerised biomass in a solvent. Filtrate is finally distilled to recover volatile solvents and to concentrate liquefied biomass (bottom).

For the results shown, by burning a sample of insoluble residue at 550°C for 6 h in air, ash content of the insoluble residue was determined.

Process can be performed batch or continuous mode.

More specifically, BiAR process deliver the complete separation of the organics, which are valid for energetic uses, from inorganics components, which are a limitation for energetic uses but however, are valuable elements that are recovered as metals.

Coming specifically to digestate case, anaerobic digestion is only capable to recover about 50% of the carbon in form of methane (according with COD balance), leaving a lot of unexploited energy in the digestate which is subsequently disposed. Thus, BiAR could be used to ‘retrofit’ those installation to recover the remaining 50% organic carbon leading to a potential 100% carbon, thus energy, recovery.

RESULTS

Sewage sludge was provided by Central Waste-Water Plant Ljubljana. After aerobic step, sludge was anaerobically treated at mesothermophilic conditions (32–37°C). Afterwards sludge was mechanically dehydrated to 35–40% of moisture; while final dehydration and granulation was performed in drying-granulation drum to 10% moisture and granulate size of 2–4 mm.

All feedstock samples were dried at 80°C overnight in an oven purged with N2 (5.0 purity) and kept in a desiccator or gas-tight flasks prior to experiments or characterization. Ash content of each sample was measured by burning organics in a furnace at 550°C in air for 6 h and weighting the mass of a leftover inorganics.

Experimental conditions were set the same for all experiments and because all samples were dried before treatment the results show different behavior based on their dry mass composition. No optimization was thus employed to specific feedstock in order to enhance higher conversion of organics, for example, or to use higher moisture feedstock.

The goal is the total theoretical conversion of organic matter and at the same time that the solid residue (filter cake) would only contain ash, specifically the whole mass of ash present in the feedstock. Total ash recovery is therefore desired by filtration, and that no ash is present in the filtrate.

From Table 2, using only MeOH as solvent, organic conversion of digestate was 85%, leading to only 15% of biomass lost in the filter cake along with ash and 91% of ash separated by filtration; while the remaining might be still present in the liquid phase (Hg, soluble salts) or lost during the manipulation. In fact small samples were used to test the process and their absolute amount of ash was small leading to experimental errors.

Table 2

Summarised laboratory results for the liquefaction of organic matter and ash recovery by filtration

Feed Ash (wt.%) Solvent Conversion of organics (wt.%) Ash recovery by filtr. (wt.%) Ash in filtrate (wt. %) 
Digestate 34.4 ± 0.8 MeOH + GLY 75 96 n.a. 
Digestate 34.4 ± 0.8 MeOH 85 91 n.a. 
Fir 0.34 ± 0.03 MeOH + GLY >99 >99 <0.07 
Fir 0.34 ± 0.03 MeOH 77 >99 <0.07 
Bark 9.1 ± 0.1 MeOH + GLY 73 42a 0.0016 
Bark 9.1 ± 0.1 MeOH + GLY 73 42a 0.29 
Rice husk 15.9 ± 0.3 MeOH + GLY 98 77a 0.023 
Feed Ash (wt.%) Solvent Conversion of organics (wt.%) Ash recovery by filtr. (wt.%) Ash in filtrate (wt. %) 
Digestate 34.4 ± 0.8 MeOH + GLY 75 96 n.a. 
Digestate 34.4 ± 0.8 MeOH 85 91 n.a. 
Fir 0.34 ± 0.03 MeOH + GLY >99 >99 <0.07 
Fir 0.34 ± 0.03 MeOH 77 >99 <0.07 
Bark 9.1 ± 0.1 MeOH + GLY 73 42a 0.0016 
Bark 9.1 ± 0.1 MeOH + GLY 73 42a 0.29 
Rice husk 15.9 ± 0.3 MeOH + GLY 98 77a 0.023 

aExperimental error might have been the cause for relatively low ash recovery.

Rice husks had very high ash content (15.9 wt.%) but their ash removal was a great success, with organic matter conversion of 98%, high degree of ash separation by filtration (ash recovery 77 wt.%) and insignificant ash content in the liquid phase (0.02%).

Finally, Table 2 shows that among tested samples, only debarked fir has suitable ash content for direct pellet production (<0.5 wt.%), while ash content in other samples exceeds the permitted value for 18 to 68 times, while after the BiAR process they meet ash limits for EN plus A1.

Digestate sludge had the highest amount of ash among the feedstock tested. Trials carried out with MeOH solvent showed higher conversion of organics (85%) compared to the case where MeOH + GLY solvent was used (75%). Opposite behaviour was observed for Fir liquefaction leading to the conclusion that MeOH solvent is to prefer for digestate and sludge liquefaction.

SOLVENT RECOVERY

Methanol recovery from the filtrate was investigated in a rotavapor, where lowering of the distillation pressure from ambient to 50 mbar at constant temperature (70°C) simulated the distillation at ambient pressure and high (variable) temperature. Since glycerol boiling temperature is as high as 290°C, distillation of pure glycerol is not of practical interest, while distillation of lower alcohols would be desirable. Up to 95% MeOH was recovered as a clear distillate (Figure 2), nevertheless, MeOH recovery over 95% was not possible, due to high boiling point of MeOH-liquefied wood system, when liquefied wood components content becomes comparable to MeOH content.

Figure 2

Distilated filtrates (dark) and distillates (transparent).

Figure 2

Distilated filtrates (dark) and distillates (transparent).

ECONOMIC CONSIDERATIONS

Albeit the process has not been optimized for any of the feedstock, we tried to give an approximate indication of the variable cost associated with this process. Depending on cost of power, thermal energy and feedstock we can estimate a cost ranging from 25 to 100 €/ton. Retail price for pellets goes from 150 to 230 €/ton, thus with significant potential added value.

In case of sludge, avoided cost of digestate disposing should also be considered, and overall, a complete compensation of disposing cost with product value can be envisaged.

CONCLUSIONS

  • 1.

    BiAR process is an innovative technology to remove inorganics from ash-rich biomass by employing solubilisation of organic matter and physically separating the ash-rich solid residues.

  • 2.

    Almost all inorganics in the sludge (34.4 wt.%) was successfully removed and the ash-free product had Gross Calorific Value (GCV) of around 21 MJ kg–1, even higher than ENplus A1 wood pellets of the best quality.

  • 3.

    Liquefaction of biomass in nearly-critical methanol is possible, although liquefaction yields are significantly increased in partial presence of glycerol.

  • 4.

    By using MeOH and GLY mixture, organic matter conversion exceeded 95% when liquefying rice husks or fir sawdust.

  • 5.

    Successful separation of the ash by liquefaction and filtration was confirmed for rice husks and digestate, while more tests would be required to close ash mass balance for bark, although ash content of the liquid phase was very low.

  • 6.

    Solvent recovery was easy by vacuum distillation.

  • 7.

    After BiAR process, all feedstocks tested meet the ash limits set by EN plus standards.

  • 8.

    Solid-liquid separation is simple by filtration.

  • 9.

    Because of the liquefaction phase involved in the process, the final product can be shaped into any liquid or solid form, e.g. 6 mm diameter EN standardized pellets without the need of any energy intensive procedure.

  • 10.

    The PCT application has been recently filed for the BiAR process (PCT/IT2016/000140) (Faussone et al. 2016).

REFERENCES

REFERENCES
Eubionet3
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New European Pellets Standard – EN 14961-1. Alakangas, Eija, VTT. New European Pellet Standard – EN 14961-1
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Faussone
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G. C.
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Grilc
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M.
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Likozar
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‘Process and related system for removing ashes from biomasses’: patentn application: PCT/IT2016/000140, 2016-05-30. München: World Intellectual Property Organization, International Bureau
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